/rofs directory and root / directory & /dev/loop0

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http://www.backtrack-linux.org – My persistent USB installation has what looks to be the same file structure in / as it does in /rofs . Right now /dev/loop0 is out of space. /dev/loop0 is mounted to /rofs I can't find info on /rofs , but since the same subdirectories exist at / seems like I could eliminate one or the other to free up space. What is /dev/loop0 ? What is /rofs? Thanks (HowTos)